# Rotating Spacecraft Causing 'Artificial Gravity' via Centripetal Force

How is this possible?

The reason spinning a bucket of water upside down keeps the water inside the bucket is because you're applying force and accelerating the bucket.

But in space, there is nothing 'accelerating' the rotation of a spacecraft, it is merely in continuous Newtonian motion, and the spacecraft and all of its contents would be at rest relative to each other wouldn't they?

I thought everything was relative, and what appears to be 'rotation' in space depends on your own frame of reference. So how would could this 'artificial gravity' occur?

phinds
Gold Member
Rotation of the type you are talking about is ALWAYS acceleration. You even say yourself that the bucket is being accelerated. Why do you see a rotating spacecraft as different from the bucket?

Rotation of the type you are talking about is ALWAYS acceleration. You even say yourself that the bucket is being accelerated. Why do you see a rotating spacecraft as different from the bucket?

Well, because what is the difference between "spinning" and standing still in space? Doesn't it all depend on frame of reference?
With the bucket, aren't you continuously applying force with your hand to keep it spinning? But in the case of a rotating spacecraft, force only had to be applied once to get it rotating - so how is acceleration constantly occurring without the application of any force? Where is the force coming from?

AlephZero
Homework Helper
The bucket analogy breaks down for two reasons. One is that on earth you have to do work to overcome air resistance slowing down the bucket, etc, and in space that doesn't apply. The other one is that a single bucket is an unbalanced system, and you have to provde ALL the centripetal force to make it travel round a circle.

A better analogy is spinning a bike wheel. This is a balanced system, because the center of mass of the complete wheel is at the axis of rotation. The wheel gradually slows down because of air resistance, but ignoring that effect (which doesn't occur in space) you don't have to apply any external forces to keep it spinning.

Each point on the rim of the wheel is always accelerating towards the centre, so as you said there most be a force acting on it to make that happen. That force comes from the internal stress in the rim of the wheel, plus the stress (tension) in the spokes. If you spun the wheel fast enough, those stresses would become big enough to break the wheel.

A rotating spacecraft "works" the same way - the internal stresses in the structure of the spacecraft provide the forces to make each part of it rotate.

For rotatiion at constant speed, those forces do not do any work. The direction of motion of each particle is along a tangent to a circle, but the force acts towards the center of the circle, at right angles to the direction of motion. In the "work = force x distance" formula, the force and distance must be measured in the SAME direction.

In addition to what was said above,
I thought everything was relative, and what appears to be 'rotation' in space depends on your own frame of reference.

This is not correct. Well, okay it is kind of, but the laws of physics are not the same if you are in the rotating frame. Relativity (Galilean and Special) applies to inertial reference frames. A spinning ship is accelerating.

That is a well explained response AlephZero.

However, if the passenger on the spacehip is not attached to the structure, then (tensile) centripetal force cannot be responsible for a force exerted on the passenger that is toward the centre/axis of rotation.

Building on the bike wheel analogy, let us firstly scale up the size significantly, and then add a tyre made of some "spaceworthy" material. Let the inside of the tyre be pressurised to 1atm. When spinning on its axis in (neg-gravity, vacuum) space, in which direction would a hypothetical passenger inside the "tyre-ship" move, and why?

The idea of artificial gravity seems to suggest that the part of the tyre wall furtherest from the axis becomes "down" - ie. if the passenger stands here, the top of their head points toward the axis of rotation.

AlephZero
Homework Helper
If the passenger is standing on the rotating structure (i.e. standing on the outside with head pointing towards the axis of rotation), the friction force on his/her feet ensures he/she has the same a tangential velocity as the spacecraft, and the normal force provides the centripetal acceleration.

But if the passenger was "floating" inside the ship, there would be nothing to make the him/her feel any artificial gravity - at least until some part of the rotating ship's structure hit him/her!

A.T.
But in the case of a rotating spacecraft, force only had to be applied once to get it rotating
Every mass in circular motion needs a non-zero net force all the time. Parts of the rotating body are exerting forces on each other.

The frictional force and the normal force is a bit of a chicken and the egg scenario isnt it? Given that the frictional force is
F = frictional coefficient x N
Where N is effectively equal to the normal force.

jbriggs444
Homework Helper
The frictional force and the normal force is a bit of a chicken and the egg scenario isnt it? Given that the frictional force is
F = frictional coefficient x N
Where N is effectively equal to the normal force.

Indeed. If you are just floating there, there is nothing to "prime the pump" and get friction started.

But all it takes is one tiny bump. And if there is air in the space station, then the air will be moving with it. That's enough to get things started as well.

1 person
I have a few more questions.

If the force maintaining the acceleration of rotation is coming from electromagnetic tension in the structure of the spacecraft itself, wouldn't friction between the particles in the structure eventually stop it from rotating, perhaps turning the tension into heat?

Also, since this source of this force is seemingly unlimited, why wouldn't we be able to harness it for energy? For example, we have a disc spinning in zero G generating energy in the same fashion as a wind turbine, then on the opposite side of the disk have a magnet apparatus with an opposite charge to balance it out so the magnet from the generator wouldn't eventually slow it down.

I know "free energy" isn't actually possible and I'm not seriously proposing it, but I'm still curious what the specific reason is for why this wouldn't work.

Nugatory
Mentor
If the force maintaining the acceleration of rotation is coming from electromagnetic tension in the structure of the spacecraft itself, wouldn't friction between the particles in the structure eventually stop it from rotating, perhaps turning the tension into heat?
No. There's no friction because there's no relative movement between the particles, despite the tension. It's no different than hanging a weight from a string - the string is under tension, but it can remain under tension more or less forever without ever doing any work or generating any heat from internal friction.

Also, since this source of this force is seemingly unlimited, why wouldn't we be able to harness it for energy? For example, we have a disc spinning in zero G generating energy in the same fashion as a wind turbine, then on the opposite side of the disk have a magnet apparatus with an opposite charge to balance it out so the magnet from the generator wouldn't eventually slow it down.

A wind turbine depends on the wind moving past it to provide the energy input. You're describing a perfectly ordinary wind-powered generator, something that's found all over the world. Zero-G conditions have nothing to do with their operation.

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1 person
No. There's no friction because there's no relative movement between the particles, despite the tension. It's no different than hanging a weight from a string - the string is under tension, but it can remain under tension more or less forever without ever doing any work or generating an heat from internal friction.
That's a really good analogy; it makes a lot of sense to me now, thanks

A wind turbine depends on the wind moving past it to provide the energy input. You're describing a perfectly ordinary wind-powered generator, something that's found all over the world. Zero-G conditions have nothing to do with their operation.
Hmm. Why doesn't NASA ever use turbines in space for power as an alternative for solar panels? You'd think that with no air resistance, they'd be able to get them spinning at extremely high speeds and generate tremendous power; plus the lifetime of a space turbine would probably be a lot longer than a solar panel, which are always taking a beating and getting damaged by debris

Hmm. Why doesn't NASA ever use turbines in space for power as an alternative for solar panels? You'd think that with no air resistance, they'd be able to get them spinning at extremely high speeds and generate tremendous power; plus the lifetime of a space turbine would probably be a lot longer than a solar panel, which are always taking a beating and getting damaged by debris

With no air resistance what would get them spinning at all? Air is used to turn the turbine. Without air flow you would need to provide something to do the turning. Your device consumes power, it doesnt produce power.

Nugatory
Mentor
Hmm. Why doesn't NASA ever use turbines in space for power as an alternative for solar panels?

There's no wind in outer space to turn the turbines.

With no air resistance what would get them spinning at all? Air is used to turn the turbine. Without air flow you would need to provide something to do the turning. Your device consumes power, it doesnt produce power.

I guess I didn't mean to use the word "turbine"; it could be anything spinning, like a disc. In 0g with no air resistance, you'd only have to start the object spinning and it would stay spinning. You use a magnet apparatus on one side of the disc to generate energy; and a magnet of opposite charge on the other side and opposite end of the turbine to prevent the first magnet from slowing down the disc.

I'm almost positive it wouldn't work, but humor me.
It just throws me off that a form of acceleration requires no energy to be maintained.

Drakkith
Staff Emeritus
I guess I didn't mean to use the word "turbine"; it could be anything spinning, like a disc. In 0g with no air resistance, you'd only have to start the object spinning and it would stay spinning. You use a magnet apparatus on one side of the disc to generate energy; and a magnet of opposite charge on the other side and opposite end of the turbine to prevent the first magnet from slowing down the disc.

I'm almost positive it wouldn't work, but humor me.
It just throws me off that a form of acceleration requires no energy to be maintained.

It would very quickly slow down from the back EMF generated.

berkeman
Mentor
I have a few more questions.

If the force maintaining the acceleration of rotation is coming from electromagnetic tension in the structure of the spacecraft itself, wouldn't friction between the particles in the structure eventually stop it from rotating, perhaps turning the tension into heat?

Also, since this source of this force is seemingly unlimited, why wouldn't we be able to harness it for energy? For example, we have a disc spinning in zero G generating energy in the same fashion as a wind turbine, then on the opposite side of the disk have a magnet apparatus with an opposite charge to balance it out so the magnet from the generator wouldn't eventually slow it down.

I know "free energy" isn't actually possible and I'm not seriously proposing it, but I'm still curious what the specific reason is for why this wouldn't work.

I guess I didn't mean to use the word "turbine"; it could be anything spinning, like a disc. In 0g with no air resistance, you'd only have to start the object spinning and it would stay spinning. You use a magnet apparatus on one side of the disc to generate energy; and a magnet of opposite charge on the other side and opposite end of the turbine to prevent the first magnet from slowing down the disc.

I'm almost positive it wouldn't work, but humor me.
It just throws me off that a form of acceleration requires no energy to be maintained.

No, we won't humor you. "Free Energy" discussions are among the banned topics for the PF. It is a waste of time to discuss/debunk it in threads. Since your original question has been answered, I will close this thread with a quote from the rules -- you can follow the links to read why your Free Energy idea does not work.

PF Rules said: